The present invention relates generally to a mixer used in fields of radio communications, measurements and so on, and more particularly to a UHF-SHF mixer for outputting an intermediate frequency signal as a result of mixing of a radio frequency signal and a local oscillator signal inputted thereinto.
Double-balanced type mixers have been developed heretofore and employed for frequency-conversion circuits in high-frequency applications from the viewpoint to many advantageous features such as excellent separation between a local oscillator signal (LO) and a radio frequency signal (RF) and little distortion. The double-balanced type mixer generally comprises a two balanced-to-unbalanced circuits, each of which can be realized by a ferrite core and a winding therearound under the condition that the frequency is below a predetermined value (IGHz), resulting in a small-sized and high-performance mixer. However, in the case of frequencies over the predetermined value, the realization of the balanced-to-unbalanced circuits with the ferrite core and winding becomes impossible because of deterioration of the characteristic of the ferrite. One approach may be to use rat race circuits for realization of the balanced-to-unbalanced circuits in the high-frequency case, the use of a rat race circuit for a mixer being illustrated in U.S Pat. Nos. 4008438 and 4031472. However, the use of the rate race circuits results in complex, high-cost and low-performance arrangement of the mixer.
Furthermore, the isolation terminals of the rat race circuits generally remains in the open-ended state so as not to completely perform the isolation of a high-frequency signal, resulting in unbalance between the rat race circuits and hence in deterioration of the isolation characteristic between the LO input terminal and RF input terminal.